Humans are increasingly subjected to ionizing radiation from a number of sources. The radiation occurs in the form of high-energy electromagnetic radiation, such as X-ray or gamma-radiation. It cannot be detected directly by humans. Depending on the type and duration of exposure to radiation, however, one's health can be damaged.
The effects of such energy-rich radiation can occur intentionally, e.g. in the medical application of ionizing rays during diagnostic or therapeutic X-ray treatment or in nuclear medicine, in non-destructive materials testing, in radiometry or in special measuring techniques using devices which contain radioactive substances, or unintentionally, e.g. when operating defective radiation emitters such as acceleration units, electron microscopes, electronic welding plant, electron tubes or monitors. When operating X-ray devices or other radiation emitting instruments, the unintended effects of radiation on the operator or third parties may occur. Therefore many precautions are applied to protect the operator or third parties from this radiation. In many cases it is then not possible to operate the devices mentioned above, or only when third parties are not present. In many cases also, complete separation of the operating staff from the source of radiation is not possible, and also not practicable, because the operator can only operate the equipment in the immediate vicinity of the equipment and the source of radiation. This applies in particular for the medical use of X-rays in X-ray diagnostics and X-ray therapy when structural radiation protective measures have to be used on the apparatus and personal protection equipment has to be used in order to screen the operating staff and/or the patients from the radiation, with the exception of areas where the radiation is actually required.
Typical radiation protection materials contain flat materials, in particular metallic lead or lead compounds or lead blends. Lead and its compounds are frequently used to protect against X-radiation and gamma-radiation. Lead has the advantage that it is available at low cost, has a high density and also has a high atomic number. It is, therefore, a good absorber of ionizing radiation, e.g. X-radiation which is produced with accelerating potentials of 40 to 300 kV. The disadvantage of lead is that, due to the photoelectric effect, the degree of attenuation of lead is relatively small at low energies of the ionizing radiation. Also, lead has questionable toxicological properties. On top of that, there is the great weight of lead-containing protective equipment.
Thus, there is a great need for materials which exhibit similarly effective screening properties with regard to ionizing radiation as those of lead but which are substantially lighter, more environmentally friendly and more toxicologically acceptable than lead.
JP 58-053828 (K. Yamamoto) describes an elastic rubber-like foam material based on polychloroprene rubber which contains large amounts (80-87.3 wt. %) of metal compounds, e.g. lead oxide.
JP-57-141430 discloses a lead-containing foamed material which consists of natural rubber or synthetic rubber and contains lead compounds at the rate of 300 or more parts by weight per 100 parts by weight of the base material.
CA-A 815 609 describes a flexible material which consists of a braided base layer and a lead-containing elastomer layer, at least one surface of which is glued to the base layer. The base layer contains lead particles with a size of <200 mesh. The lead makes up at least 65 wt. % of the total weight of the material. The preferred elastomer material is neoprene (polychloroprene).
JP 61-228051 discloses compositions of ethylene/vinyl acetate and/or ethylene/ethyl acrylate copolymers which contain 5 to 50 parts of antimony oxide and 5 to 100 parts of barium sulfate per 100 parts of polymer as cable casing. The disadvantage of this composition is the high proportion of antimony oxide which is classified as a carcinogenic compound.
Compositions of metallic lead in polyvinylchloride for the absorption of X-radiation are described in GB-A 1 603 654 and GB-A 1 603 655.
JP 59-126296 describes a coated composition for screening against radiation, which contains lead or lead compounds in a copolymer and is applied to plasticized polyvinylchloride.
A flexible material for screening against radiation, consisting of an elastomer matrix which contains a homogeneous distribution of filler particles, is described in GB-A 1 122 786. The filler is formed from a mixture of ionizing radiation absorbing metal and at least one other metal. Lead and lead/antimony alloys are used in this case.
GB-A 954 593 describes screenings against ionizing radiation which are presented in the form of lead coated fabrics which have been immersed in mercury and thereby form lead amalgams, and thus improve the flexibility of the coated fabric.
Radiation screening materials which also contain a lead-containing methacrylate plastics material are disclosed in JP-2360/1960, JP-9994/1978, JP-9995/1978, JP-9996/1978 and JP-63310/1978.
EP-A 371 699 discloses radiation screening materials which, in a preferred embodiment, are an inorganic mixture of lead, actinium, bismuth, gold, mercury, polonium, thallium, thorium, uranium, iridium, osmium, platinum, rhenium, tantalum, tungsten, bromine, molybdenum, rhodium, strontium or zirconium and inter alia cerium or lanthanum. 70 to 93 wt. %, or preferably in fact 70 to 90 wt. % of the mixture is used in copolymers of ethylene and alkyl acrylate, alkyl methacrylate, glycidyl methacrylate, acrylic acid, methacrylic acid. In the preferred embodiment, 5 to 10 wt. % of plasticizer is also added.
U.S. Pat. No. 4,563,494 describes lanthanoid compounds which can be used in polyacrylates, polymethacrylates, polystyrene and copolymers thereof in amounts of 0.001 to 10 wt. %. In that document it is expressly pointed out that complete screening against X-ray or gamma radiation is possible only in combination with lead compounds (column 5; lines 57 to 61).
In both DE-A 199 55 192 and EP-A 0 371 699 powdered metals with high atomic numbers are referred to as X-radiation absorbing fillers in elastomers, wherein high proportions of metallic tin of 50-100 wt. % are cited in particular. The use of at least 26 wt. % of gadolinium from gadolinium oxide is not disclosed.
GB-A 943 714 describes compositions for preparing a material which screens against X-radiation and which consists of a silicone elastomer with powdered tungsten as an additive.
In particular in the region of high-energy X-radiation of 90 kV to 150 kV accelerating potential, the materials provided in DE-A 199 55 192 and GB-A 943 714 do not offer any weight advantage, for the same screening properties, over lead.
Thus, in all the known processes, either lead or antimony or their compounds in high concentration, or ecologically unacceptable substances such as mercury, polonium or uranium and/or substance mixtures with high basic proportions of metal such as e.g. antimony, are used. Often, the radiation protection substances used in the known processes do not adequately screen against high-energy X-radiation in the region of 90 to 150 kV accelerating potential.
The object of the present invention is to provide, as compared with the prior art and the still preferably used lead-containing materials, lighter, more toxicologically acceptable, completely lead-free mixtures which screen against ionizing radiation, such as e.g. X-radiation or gamma radiation, better than lead.